Method for forming a titania-coated inorganic particle

ABSTRACT

A method of forming a titania-coated inorganic particle comprising the steps of (a) stirring a mixture of a titania precursor such as a titanium alkoxide and an inorganic particle such as a hollow glass particles in an organic solvent such as an alcohol for more than 1 h to cause adsorption of the titania precursor on the surface of the inorganic particle; and (b) adding water dropwise to the mixture under stirring to convert the titania precursor to titania which then forms a coating on the inorganic particle. A method for forming a paint formulation, a titania-coated inorganic particle, a paint formulation comprising a titania-coated inorganic particle and use of a titania-coated inorganic particle in a paint formulation is also described.

Cross Reference to Related Applications

This patent application is a U.S. National Phase Application Under 35U.S.C. § 371 of International Application No. PCT/SG2017/050406, filedon 16 Aug. 2017, entitled A METHOD FOR FORMING A TITANIA-COATEDPARTICLE, which claims priority from Singapore Patent Application No.10201606951U filed on 19 Aug. 2016.

TECHNICAL FIELD

The present invention generally relates to a method for forming atitania-coated inorganic particle. The present invention also relates toa method for forming a formulation comprising a titania-coated inorganicparticle.

BACKGROUND ART

It is reported that 15% of all electricity consumed worldwide is used tocool buildings. Considering the energy and environment sustainability,it is very important to decrease such electricity consumption. For theexisting buildings, the most feasible way is to use cool paint to coatthe roof and walls to reduce the electricity consumption for coolingbuildings. The principle of cool paint can be summarized in thefollowing details. When a building surface is exposed to sunirradiation, the surface undergoes solar irradiation absorption heating,solar irradiation reflectance cooling, surface emissivity cooling, andthermal conduction to the interior surface. Therefore, paint possessinghigh solar light reflectance, high thermal emissivity, and low thermalconductivity can be used as cool paint to decrease the electricityconsumed for cooling building. In order to confer the above propertiesto the paint, functional pigments are usually added to paint to allowthe paint to function as desired. Therefore, proper and effective coolpigment is important for cool paint. While a number of pigments areknown in the art, they suffer from a number of limitations, for example,aerogel (such as silica aerogel, clay aerogel etc) that have low thermalconductivity and can be used as thermal insulation panels, suffer frompoor mechanical strength and cannot withstand mixing during paintformulation. In addition, such aerogels also suffer from high synthesiscosts due to stringent requirements needed during drying. In addition,for phase change materials (such as organic materials, inorganicmaterials or eutectics) that are typically used widely in sleeping bags,clothing and electronics, they suffer from the need for encapsulationand require a minimal thickness to work.

There is thus a need to synthesize cool pigments that possess highemissivity, high solar light reflectance and/or low thermalconductivity.

There is a need to provide a method for forming a pigment thatovercomes, or at least ameliorates, one or more of the disadvantagesdescribed above.

There is a need to provide a method for forming a formulation thatcomprises a pigment that overcomes, or at least ameliorates, one or moreof the disadvantages described above.

SUMMARY

According to a first aspect, there is provided a method for forming atitania-coated inorganic particle comprising the steps of (a) stirring amixture of a titania precursor and an inorganic particle in an organicsolvent for a time period of more than 1 hour to cause adsorption of thetitania precursor on the surface of the inorganic particle; and (b)adding water to the mixture under stirring to convert the titaniaprecursor to titania which then forms a coating on the inorganicparticle.

According to a second aspect, there is provided a method for forming apaint formulation, comprising the steps of (a) stirring a mixture of atitania precursor and an inorganic particle in an organic solvent for atime period of more than 1 hour to cause adsorption of said titaniaprecursor on the surface of said inorganic particle; (b) adding water tothe mixture under stirring to convert said titania precursor to titaniawhich then forms a coating on said inorganic particle; (c) separatingthe titania-coating inorganic particle from the mixture; and (d) addingsaid titania-coating inorganic particle to a paint formulation.

The disclosed method of forming the titania-coated inorganic particle isdistinguished from the physical method of the prior art in whichdiscrete titania (or titanium dioxide) particles and inorganic particles(in the form of hollow glass beads) are mixed together and physicallystirred. The disclosed method thus does not suffer from thedisadvantages associated with the physical method such as the largedensity difference between hollow glass beads (which can be as low as0.1 g/mL) and titanium dioxide (which is 4.23 g/mL); the agglomerationof titanium dioxide powder; and the low refractive index of hollow glassbeads. The former two disadvantages will affect the paint formulationquality and additional technology will be needed to overcome the formertwo disadvantages during paint formulation. For the last disadvantage,the interface between hollow glass bead and binder is not utilizedefficiently, which limits the solar light reflectance of paint.

Advantageously, the present method of forming the titania-coatedinorganic particles may solve the above disadvantages. Thetitania-coating on the inorganic particle may be present on a large areaof the surface of the inorganic particle, such as at least 90% (90%,91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100%) of the surface ofthe inorganic particle is coated with the titania-coating. Thetitania-coating may completely cover the surface of the inorganicparticle (that is, a 100% coating coverage). Having this high coverageof the coating on the surface of the inorganic particle may lead to anincrease in the interfacial area with refractive index difference, whichwill increase solar light reflectance of the titania-coated inorganicparticles. Depending on whether the inorganic particle is hydrophilic orhydrophobic, the type of bonding between the titania and the inorganicparticle will be different. Where the inorganic particle is hydrophilic(or having a hydrophilic treated surface), the present method may beconsidered as a chemical method that results in the formation ofcovalent bonds between the titania and the surface of the inorganicparticle. Where the inorganic particle is hydrophobic (or having ahydrophobic treated surface), the present method may be considered to bea physical method that results in physical bonding between the titaniaand the surface of the inorganic particle. The present method results intitania-coated inorganic particles with high solar light reflectance,without any agglomeration of the titania particles. When thetitania-coated inorganic particle of the disclosure is used in a paintformulation, due to the increases in the refractive index difference andeffective interfacial surface area in the interface between theinorganic particle and the binder, this may lead to an increase in thesolar light reflectance.

Advantageously, the density of the titania-coated inorganic particle ofthe present application may be tuned. Due to the shape of the inorganicparticle, which is typically spherical or substantially spherical, theinorganic particle has good fluidity in paint. Thus, the titania-coatedinorganic particle will also have good fluidity in paint since thetitanit coating does not substantially change the shape of the inorganicparticle.

Advantageously, the disclosed method may be undertaken at a neutral pHand ambient conditions. This is a departure from some of the prior artmethods which rely heavily on pH control (either highly acidic or highlybasic), on temperature control (heating or cooling), surfactantutilization, precipitator utilization, preheating or post annealingtreatment of the materials to form the titania from a particularprecursor. These controls and treatments usually result in moreoperation procedures and equipments. In addition, some of the prior artmethods require the titania to be in a particular crystal phase to be ofuse. Hence, the disclosed method may optionally not include anypre-heating step. The disclosed method may optionally not require theuse of surfactants in the method. The disclosed method may be widelyapplicable to titania of any crystal phase and is not limited torequiring the titania to be in a particular crystal phase to be of use.

Advantageously, the disclosed method is more economical than prior artmethods while allowing the ability to finely tune the coating conditionsto coat the titania coating onto the surface of the inorganic particleswithout any obvious freestanding titania agglomerates to increase thesolar light reflectance property of the titania-coated inorganicparticle pigment.

According to a third aspect, there is provided a titania-coatedinorganic particle, wherein the titania is amorphous titania.

An advantage of amorphous titania over anatase titania is that theamorphous titania does not have photocatalytic performance, which willnot cause photodegradation of binder during application (such as in apaint formulation). Hence, when the amorphous titania-coated inorganicparticle is used in a paint formulation, as compared to using anatasetitania-hollow glass beads, the amorphous titania-coated inorganicparticle may aid to increase the stability of the paint while havingcomparable solar light reflectance with that of anatase titania-hollowglass beads.

According to a fourth aspect, there is provided a paint formulationcomprising a titania-coated inorganic particle, wherein the titania isamorphous titania.

According to a fifth aspect, there is provided use of a titania-coatedinorganic particle in a paint formulation, wherein the titania isamorphous titania.

Definitions

The following words and terms used herein shall have the meaningindicated: The term ‘titania’ is to be used interchangeably withtitanium dioxide, or titanium (IV) oxide, and has the chemical formulaTiO₂. Titania can exist in the amorphous form, or may be in a crystalform such as rutile, anatase, brookite, or mixtures thereof. Titania canexist as a mixture of amorphous or crystal forms, or a mixture ofdifferent amorphous forms, or a mixture of different crystal forms.

The word “substantially” does not exclude “completely” e.g. acomposition which is “substantially free” from Y may be completely freefrom Y. Where necessary, the word “substantially” may be omitted fromthe definition of the invention.

Unless specified otherwise, the terms “comprising” and “comprise”, andgrammatical variants thereof, are intended to represent “open” or“inclusive” language such that they include recited elements but alsopermit inclusion of additional, unrecited elements.

As used herein, the term “about”, in the context of concentrations ofcomponents of the formulations, typically means+/−5% of the statedvalue, more typically+/−4% of the stated value, more typically+/−3% ofthe stated value, more typically, +/−2% of the stated value, even moretypically+/−1% of the stated value, and even more typically+/−0.5% ofthe stated value.

Throughout this disclosure, certain embodiments may be disclosed in arange format. It should be understood that the description in rangeformat is merely for convenience and brevity and should not be construedas an inflexible limitation on the scope of the disclosed ranges.Accordingly, the description of a range should be considered to havespecifically disclosed all the possible sub-ranges as well as individualnumerical values within that range. For example, description of a rangesuch as from 1 to 6 should be considered to have specifically disclosedsub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4,from 2 to 6, from 3 to 6 etc., as well as individual numbers within thatrange, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of thebreadth of the range.

Certain embodiments may also be described broadly and genericallyherein. Each of the narrower species and subgeneric groupings fallingwithin the generic disclosure also form part of the disclosure. Thisincludes the generic description of the embodiments with a proviso ornegative limitation removing any subject matter from the genus,regardless of whether or not the excised material is specificallyrecited herein.

DETAILED DISCLOSURE OF EMBODIMENTS

Exemplary, non-limiting embodiments of method for forming atitania-coated inorganic particle will now be disclosed. The method forforming a titania-coated inorganic particle comprises the steps of (a)stirring a mixture of a titania precursor and an inorganic particle inan organic solvent for a time period of more than 1 hour to causeadsorption of said titania precursor on the surface of said inorganicparticle; and (b) adding water to the mixture under stirring to convertsaid titania precursor to titania which then forms a coating on saidinorganic particle.

Advantageously, the disclosed method results in the formation of atitania-coated inorganic particle that may have high emissivity, highsolar light reflectance and/or low thermal conductivity. Thetitania-coated inorganic particle may have a reflectance higher thanabout 80%, higher than about 81%, higher than about 82%, higher thanabout 83%, higher than about 84%, higher than about 85%, higher thanabout 86%, higher than about 87%, higher than about 88%, higher thanabout 89%, higher than about 90%, higher than about 91%, higher thanabout 92%, higher than about 93%, higher than about 94%, higher thanabout 95%, higher than about 96%, higher than about 97%, higher thanabout 98%, or higher than about 99%. The titania-coated inorganicparticle may have an emissivity of higher than about 0.80, higher thanabout 0.85, higher than about 0.90, higher than about 0.95, or about1.00. The titania-coated inorganic particle may have a thermalconductivity lower than about 0.3 W/(mK), lower than about 0.2 W/(mK) orlower than about 0.1 W/(mK). This may be due to the high refractiveindex of the titania coupled with the low thermal conductivity of theinorganic particle.

As compared to the prior art methods, the disclosed method may notrequire the use of pH or temperature control. Instead, the disclosedmethod relies on controlling the concentration/amount ratio of thetitania precursor, inorganic particle and water. The ratio of thetitania precursor (molar) to inorganic particle (g) may be in the rangefrom about 8.0:1 to about 18.0:1 (mmol/g), about 9.0:1 to about 18.0:1(mmol/g), about 10.0:1 to about 18.0:1 (mmol/g), about 11.0:1 to about18.0:1 (mmol/g), about 12.0:1 to about 18.0:1 (mmol/g), about 13.0:1 toabout 18.0:1 (mmol/g), about 14.0:1 to about 18.0:1 (mmol/g), about15.0:1 to about 18.0:1 (mmol/g), about 16.0:1 to about 18.0:1 (mmol/g),about 17.0:1 to about 18.0:1 (mmol/g), about 8.4:1 to about 18.0:1(mmol/g), about 8.0:1 to about 9.0:1 (mmol/g), about 8.0:1 to about10.0:1 (mmol/g), about 8.0:1 to about 11.0:1 (mmol/g), about 8.0:1 toabout 12.0:1 (mmol/g), about 8.0:1 to about 13.0:1 (mmol/g), about 8.0:1to about 14.0:1 (mmol/g), about 8.0:1 to about 15.0:1 (mmol/g), about8.0:1 to about 16.0:1 (mmol/g), about 8.0:1 to about 17.0:1 (mmol/g),about 13.0:1 to about 14.0:1 (mmol/g), about 15.0:1 to about 16.0:1(mmol/g) or about 17.0:1 to about 18.0:1 (mmol/g). In some parts of thedescription, the ratio may simply be stated as 8 to 18 mmol/g or 8.4 to18 mmol/g. It is to be appreciated that the above ranges should beinterpreted as including and supporting any sub-ranges or discretevalues (which may or may not be a whole number) that are within thestated range(s).

The molar ratio of water to titania precursor may be in the range ofabout 2:1 to about 8:1, about 2:1 to about 7:1, about 2:1 to about 6:1,about 2:1 to about 5:1, about 2:1 to about 4:1, about 2:1 to about 3:1,about 3:1 to about 8:1, about 4:1 to about 8:1, about 5:1 to about 8:1,about 6:1 to about 8:1, or about 7:1 to about 8:1. It is to beappreciated that the above ranges should be interpreted as including andsupporting any sub-ranges or discrete values (which may or may not be awhole number) that are within the stated range(s).

By using a mixing (or stirring) time of more than 1 hour, this may aidin facilitating adsorption of the titania precursor onto the surface ofthe inorganic particle. When the ratio of the various components arecontrolled together with the mixing (or stirring time), these parametersensure that the titania is coated onto the surface of the inorganicparticle without the formation of freestanding titania particles.

The titania precursor may be a titanium alkoxide. The titanium alkoxidemay be a titanium C₁₋₁₀-alkoxide. The titanium C₁₋₁₀-alkoxide may be aC₁₋₄-alkoxide selected from the group consisting of titanium methoxide,titanium ethoxide, titanium isopropoxide and titanium butoxide.

The inorganic particle may be a glass particle selected from the groupconsisting of a silica glass particle, a soda-lime glass particle, aborosilicate glass particle, an aluminosilicate glass particle andmixtures thereof. The glass particle may be a hollow glass particlecomprising a glass shell encapsulating an inner void. The inner void maycontain air, gas or a vacuum.

The inorganic particle may be of any shape and may typically be aspherical particle. The inorganic particle may be a nanosphere or amicrosphere, having a diameter (or equivalent diameter if the inorganicparticle is not an exact sphere) that is not particularly limited. Thediameter (or equivalent diameter) may be in the range of about 1nanometer to about 1000 micrometers, about 1 nanometer to about 100nanometers, about 100 nanometers to about 1 micrometer, about 1micrometer to about 100 micrometers, about 100 micrometers to about 200micrometers, or about 50 micrometers to about 100 micrometers.

The inorganic particle may have high mechanical strength, which may bedefined by having a crush strength of more than 250 psi. The crushstrength of the inorganic particle may be in the range of about 250 psito about 28,000 psi, such as about 250 psi, about 300 psi, about 400psi, about 500 psi, about 750 psi, about 2,000 psi, about 3,000 psi,about 4,000 psi, about 5,500 psi, about 6,000 psi, about 10,000 psi,about 18,000 psi, or about 28,000 psi (and ranges therebetween).

The organic solvent used may be an alcohol. The alcohol solvent is onethat is not particularly limited as long as it is miscible with anaqueous solution (such as water) and allow for good dispersion with theinorganic particles (be it hydrophobic or hydrophilic inorganicparticles). An exemplary alcohol solvent may be ethanol or isopropanol.

The titania precursor may be added to the organic solvent first at theratio mentioned above to form a solution, followed by the addition ofthe inorganic particle to form a suspension. The resultant suspensionmixture may be stirred for more than one hour, or more than two hours,or for two hours. The resultant suspension mixture may be stirred at anystirring speed that can ensure good mixture of the suspension. Exemplarystirring speed can be about 200 rpm to about 300 rpm.

After stirring (step (a)), water is added to the mixture (or suspensionmixture) under stirring in a controlled manner. The water may be addedto the mixture drop-wise (or as water droplets). During the addition ofthe water, the suspension mixture is stirred. Here, the stirring in step(b) is undertaken for more than one hour, or more than two hours, or fortwo hours.

The method may be undertaken at neutral pH. The method may not requireexplicit control of the pH or addition of pH control agents such as anacid or a base. The method may be conducted at the pH of the solventused in the method. The pH may be about 7 (+0.5).

The method may be undertaken at ambient temperatures, such as at roomtemperature (of about 25° C. to about 30° C.). The method may thus notrequire any additional heating or cooling steps in order to control thetemperature. The method may also not require the use of pre-heating,which is used in a prior art method to remove physically absorbed waterfrom the surface of the inorganic particle.

The method may further comprise the step of (c) separating thetitania-coated inorganic particle from the (suspension) mixture. Theseparating step may be a filtering step which would be known to a personskilled in the art. The filtered particle may be subjected to a washingstep with water or an alcohol to remove any excess titania precursor.

The method may further comprise the step of (d) drying thetitania-coated inorganic particle (that is filtered and/or washed) at atemperature in the range of 25° C. to 100° C. The drying step isundertaken to remove the washing medium (such as water or alcohol). Themethod may optionally not require heating the dried titania-coatedinorganic particle at a temperature above 500° C. in order to obtain adesired crystal form of the titania or to anneal the titania coating tothe inorganic particle.

The disclosed method may form a titania-coated inorganic particle havinga titania coating thickness of about 50 nm to about 300 nm, about 50 nmto about 100 nm, about 50 nm to about 150 nm, about 50 nm to about 200nm, about 50 nm to about 250 nm, about 100 nm to about 300 nm, about 150nm to about 300 nm, about 200 nm to about 300 nm, or about 250 nm toabout 300 nm. The thickness may be about 200 nm. The density of thetitania-coated inorganic particle may be tuned or controlled by thedisclosed method by controlling the ratio of titania precursor to theinorganic particle, the ratio of water to the titania precursor or thetype of washing medium. The density of the titania-coated inorganicparticle may be in the range of about 0.10 g/mL to about 1 g/mL, about0.10 g/mL to about 0.9 g/mL, about 0.10 g/mL to about 0.8 g/mL, about0.10 g/mL to about 0.7 g/mL, about 0.10 g/mL to about 0.6 g/mL, about0.10 g/mL to about 0.5 g/mL, about 0.10 g/mL to about 0.4 g/mL, about0.10 g/mL to about 0.3 g/mL, about 0.10 g/mL to about 0.2 g/mL, about0.20 g/mL to about 1 g/mL, about 0.30 g/mL to about 1 g/mL, about 0.40g/mL to about 1 g/mL, about 0.50 g/mL to about 1 g/mL, about 0.60 g/mLto about 1 g/mL, about 0.70 g/mL to about 1 g/mL, about 0.80 g/mL toabout 1 g/mL, about 0.90 g/mL to about 1 g/mL, about 0.15 to about 0.30g/mL, about 0.15 to about 0.20 g/mL, about 0.15 to about 0.25 g/mL,about 0.20 to about 0.30 g/mL, about 0.25 to about 0.30 g/mL, about 0.19to about 0.20 g/mL, or about 0.26 to about 0.27 g/mL.

Exemplary, non-limiting embodiments of method for forming a paintformulation will now be disclosed. The method for forming a paintformulation comprises the steps of: (a) stirring a mixture of a titaniaprecursor and an inorganic particle in an organic solvent for a timeperiod of more than 1 hour to cause adsorption of said titania precursoron the surface of said inorganic particle; (b) adding water to themixture under stirring to convert said titania precursor to titaniawhich then forms a coating on said inorganic particle; (c) separatingthe titania-coating inorganic particle from the mixture; and (d) addingsaid titania-coating inorganic particle to a paint formulation.

Steps (a) to (c) of this method are substantially similar to the steps(a) to (c) of the method for forming a titania-coated inorganic particlementioned above and the same conditions/criteria apply here as well.

The titania-coated inorganic particle may be added to the paintformulation at a weight % of about 1 to about 20 wt %, about 1 to about5 wt %, about 1 to about 10 wt %, about 1 to about 15 wt %, about 5 toabout 20 wt %, about 10 to about 20 wt % or about 15 to about 20 wt %.As is known in the art, paint typically contains four essentialingredients, such as pigment, binder, liquid and additives. The paintformulation is one that is not particularly limited and can be any paintformulation that requires enhancement to the solar heat reflectance andthermal insulation properties.

There is provided a titania-coated inorganic particle, wherein thetitania is amorphous titania. The titania coating may have a thickness,density and coverage as mentioned above. There is provided a paintformulation comprising a titania-coated inorganic particle, wherein thetitania is amorphous titania. The titania-coated inorganic particle maybe present in the paint formulation at a weight % as defined above.There is also provided use of a titania-coated inorganic particle in apaint formulation, wherein the titania is amorphous titania.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings illustrate a disclosed embodiment and servesto explain the principles of the disclosed embodiment. It is to beunderstood, however, that the drawings are designed for purposes ofillustration only, and not as a definition of the limits of theinvention.

FIG. 1

FIG. 1 is a reaction scheme showing the formation of a titania coatingon the surface of a (a) hydrophilic hollow glass bead; and (b)hydrophobic hollow glass bead.

FIG. 2

FIG. 2 is a series of scanning electron microscopy (SEM) images of (a)K1 hollow glass beads at a magnification of ×350 and (b) thecorresponding film after brush coating at a magnification of ×100.

FIG. 3

FIG. 3 is a series of SEM images of (a) K25 hollow glass beads at amagnification of ×100 and (b) the corresponding film after brush coatingat a magnification of ×100.

FIG. 4

FIG. 4 is a series of elemental mapping images of titania-coated hollowglass beads where (a) and (b) are based on the sampleHGB@TiO₂-3.1/2:1/ethanol/80 (at a scale of 100 μm) and (c) and (d) arebased on the sample HGB@TiO₂-4.65/2:1/ethanol/80 (at a scale of 50 μm).

FIG. 5

FIG. 5 is a series of SEM images of (a) K25 hollow glass beads at amagnification of ×100, (b) HGB@TiO₂-3.1/2:1/ethanol/80 at amagnification of ×100, (c) HGB@TiO₂-4.65/2:1/ethanol/80 at amagnification of ×100, and (d) HGB@TiO₂-4.65/2:1/water/80 at amagnification of ×100.

FIG. 6

FIG. 6 is a series of SEM cross-sectional images of (a) hollow glassbeads at a magnification of ×7,000, (b) hollow glass beads at amagnification of ×9,000, (c) HGB@TiO₂-4.65/2:1/water/80 at amagnification of ×15,000 and (d) HGB@TiO₂-4.65/2:1/water/80 at amagnification of ×16,000.

FIG. 7

FIG. 7 is a schematic diagram illustrating a self-made temperaturedifference test box.

FIG. 8

FIG. 8 is a series of SEM images of (a) hydrophobic hollow glass bead(uncoated) at a magnification of ×100, with the inset at a magnificationof ×1,000, and (b) HGB(Hydrophobic)@TiO₂-4.96/2:1/ethanol/80 at a scaleof 0.5 mm, with the inset at a scale of 25 μm.

FIG. 9

FIG. 9 is a series of elemental mapping images of (a), (b)HGB(Hydrophobic)@TiO₂-4.96/2:1/ethanol/80, both at a scale of 30 μm.

FIG. 10

FIG. 10 is a series of photographs showing the dispersion of (a)original hydrophobic hollow glass beads and (b) titania-coatedhydrophobic hollow glass beads in water.

FIG. 11

FIG. 11 is a graph showing the X-Ray Diffraction pattern oftitania-coated hollow glass bead.

DETAILED DESCRIPTION OF DRAWINGS

Referring to FIG. 1, there is provided a reaction scheme showing theformation of a titania coating on the surface of a (a) hydrophilichollow glass bead; and (b) hydrophobic hollow glass bead.

As shown in FIG. 1(a), the surface of a hydrophilic hollow glass bead 1is shown. When the titania precursor such as titanium alkoxide is addedto the hydrophilic hollow glass bead 1, after a sufficient long mixingtime, the alkoxy group in titanium alkoxide may facilitate adsorption oftitanium alkoxide on the surface of hydrophilic hollow glass bead 1 byreacting with the surface hydroxyl (—OH) groups to form —Ti—O-(hollowglass bead) covalent bonds (as seen in 2). When a controlled amount ofwater is added, hydrolysis occurs, converting some of the alkoxidegroups to hydroxyl groups (as seen in 4). Polycondensation then occurswhere titania seeds are formed on the surface of the hollow glass beaddue to the relatively high concentration of titanium alkoxide on thesurface of the hollow glass beads as compared with that in solution (asseen in 6). The titania seeds then facilitate the titania film formationon the surface of the hollow glass bead (as seen in 8).

As seen in FIG. 1(b), the surface of a hydrophobic hollow glass bead 1′is shown. When the titania precursor such as titanium alkoxide is addedto the hydrophobic hollow glass bead 1′, after a sufficient long mixingtime, the alkoxy group in titanium alkoxide may facilitate adsorption oftitanium alkoxide on the surface of hydrophobic hollow glass bead 1′ (asseen in 2′) by physical bonds. When a controlled amount of water isadded, hydrolysis occurs, converting some of the alkoxide groups tohydroxyl groups (as seen in 4′). Polycondensation then occurs wheretitania seeds are formed on the surface of the hollow glass bead due tothe relatively high concentration of titanium alkoxide on the surface ofthe hollow glass beads as compared with that in solution (as seen in6′). The titania seeds then facilitate the titania film formation on thesurface of the hydrophobic hollow glass bead (as seen in 8′).

Thus, the titanium alkoxides undergo hydrolysis (to form Ti—O—H bonds)and polycondensation to form three-dimensional structures with Ti—O—Tibond when reacted with water. The morphology of titania is highlydependent on the relative reaction rate of hydrolysis andpolycondensation, which is also strongly affected by the concentrationof water. In order to achieve full coverage of hollow glass bead withtitania, this requires careful control of mixing time, the molar ratioof water to titania precursor, and ratio of the titania precursor to theinorganic particle.

EXAMPLES

Non-limiting examples of the invention and a comparative example will befurther described in greater detail by reference to specific Examples,which should not be construed as in any way limiting the scope of theinvention.

Example 1—Coating Titania on Surface of Hollow Glass Bead

Tetrabutyl titanate (obtained from Sigma Aldrich of St. Louis ofMissouri of the United States of America) was added to 72 mL ofanhydrous ethanol. 0.6 g of K25 hollow glass bead (obtained from 3MCompany of Minnesota of the United States of America), which ishydrophilic, was added into the solution with ratio of titanium alkoxide(molar) to hollow glass bead (g) in the range from 8 to 18 mmol/g andstirred for 2 hours. Water was added drop wise to the suspension at amolar ratio of water to tetrabutyl titanate in the range of 2:1 to 8:1and stirred mechanically for 2 hours. The suspension was filtered andthe titania coated hollow glass beads washed with water or ethanol. Thetitania coated hollow glass beads were dried at room temperature (ofabout 25° C.) or at temperature lower than 100° C. The titania coatedhollow glass bead samples were collected and named as HGB@TiO₂-A/B/C/D,where A represents the ratio of tetrabutyl titanate to hollow glass beadwith unit of mL/g, B represents the molar ratio of water to tetrabutyltitanate, C represents the substance used to wash the sample, and Drepresents the temperature used to dry the sample.

For coating titania or TiO₂ onto the surface of hydrophobic hollow glassbead, the same process and parameters are utilized, and the sample isnamed HGB(hydrophobic)@TiO₂-A/B/C/D.

Example 2—Paint Formulation Example 2a

HGB@TiO₂-4.65/4:1/water/80 was added to binder A form paint. Binder A isa mixture of copolymer (which is Acronal® S 400, obtained from BASF SEof Ludwigshafen of Germany) and 20 wt % of calcium carbonate. The paintwas coated onto a glass substrate. After each coating, the wet film wasdried at room temperature (of about 25° C.) for 24 hours. The coatingand drying operations were repeated. The thickness of the dry filmranged between 0.1 mm and 1 mm based on the coating times and wet filmthickness control.

Example 2b

the same steps as Example 2a was carried out here, but using sample HGB@TiO₂-4.65/2:1/water/80.

Example 2c

Pigments were added to binder A to form paint. The pigment is one of thefollowing pigments: hollow glass bead, hollow glass bead and TiO₂physical mixture, TiO₂ coated hollow glass bead, andHGB@TiO₂-4.65/2:1/water/80. The paint was coated onto the surface of abiaxially oriented polypropylene (BOPP) film. After each coating, thewet film was dried at room temperature (of about 25° C.) for 24 hours.The coating and drying operations were repeated. The thickness of thedry film was around 1 mm. The BOPP film was peeled off to get thefreestanding coating for thermal conductivity test.

Example 2d

Pigments were added to binder B, which is Acronal® S 400 to form paint.The paint was coated onto a cement substrate. After each coating, thewet film was dried at room temperature (of about 25° C.) for 24 hours.The coating and drying operations were repeated. The thickness of thedry film ranged between 0.8 mm and 1 mm based on the coating times andwet film thickness control.

Example 2e

Hollow glass bead K1 or K25 (3M) were added to Binder B Acronal® S 400such that the concentration of the hollow glass bead in paint is 10 w %.This suspension was stirred, then the suspension was brush-coated on thesurface of a BOPP film. The film was dried at room temperature for SEMtest.

Example 3—Characterization and Performance Test of Sample

Scanning electron microscopy (SEM, JEOL LV SEM 6360LA) and energydispersive spectroscopy (JEOL JED-2300 EDX) were used to test thesurface morphology, TiO₂ dispersion, and TiO₂ layer thickness of TiO₂coated hollow glass beads. Optima 5300 DV inductively coupled plasmaoptical emission spectrometer (ICP-OES, Perkin-Elmer) was used to testthe concentration of TiO₂ in the TiO₂ coated hollow glass bead.UV—VIS-NIR spectrophotometer UV-3600 (Shimadzu) with integrating sphereISR 3100 was used to test the diffusive solar reflectance of thepigment.

FIG. 2 shows the SEM images of (a) K1 hollow glass beads and (b) thecorresponding film after brush coating as mentioned in Example 2e. Thehollow glass beads possess isostatic crush strength of 250 psi andthermal conductivity of 0.047 W/(m·K).

FIG. 3 shows the SEM images of (a) K25 hollow glass beads and (b) thecorresponding film after brush coating as mentioned in Example 2e. Thehollow glass beads possess isostatic crush strength of 750 psi andthermal conductivity of 0.085 W/(m·K).

In order to make use of the thermal insulation property of hollow glassbeads, it is necessary to maintain the integrity of the hollow glassbead after coating. Brush coating is the commonly used method to coatpaint onto a surface. FIG. 2(b) shows that after brush coating, some ofthe hollow glass beads with isostatic crush strength of 250 psi werebroken. FIG. 3(b) shows that hollow glass beads with isostatic crushstrength of 750 psi can withstand the brush coating. Therefore, K25hollow glass beads were selected for TiO₂ coated hollow glass beadsynthesis.

FIG. 4 shows the results of elemental mapping using SEM EDS where theTiO₂ coating on the surface of hollow glass beads was characterized.FIG. 4 shows that TiO₂ can be coated onto hollow glass bead by using thedisclosed method for samples HGB@TiO₂-3.1/2:1/ethanol/80 (FIG. 4a ) andof HGB@TiO₂-4.65/2:1/ethanol/80 (FIG. 4b ).

FIG. 5 shows the surface morphologies of TiO₂ coated hollow glass beads.It can be seen that TiO₂ can be coated onto the surface of hollow glassbeads without obvious freestanding TiO₂ agglomerate formation (whencomparing FIG. 5b , FIG. 5c and FIG. 5d to FIG. 5a ). The ratio oftitanium alkoxide (molar) to hollow glass bead (g) was controlled in therange from 8.4 to 18 mmol/g. The molar ratio of water to titaniumalkoxide was controlled in the range from 2:1 to 8:1.

FIG. 6 shows the cross sectional SEM images for TiO₂ coated hollow glassbead confirming the formation of a TiO₂ layer onto the surface of hollowglass beads. The thickness of the TiO₂ layer is around 162 nm in FIG. 6cand 231 nm in FIG. 6 d.

Table 1 below shows the solar light reflectance property of the varioussamples based on the measurement of diffusive solar light reflectance ofthe sample powder. The results show that the samples washed by watershowed higher diffusive solar light reflectance compared to that ofsamples washed by ethanol. For samples washed with ethanol, samplesdried at room temperature showed higher performance compared to that ofsamples dried at 50° C. and 80° C. For samples washed with water, whenthe drying temperature was higher than 80° C., the diffusive solar lightreflectance decreased. Therefore, drying temperature lower than 100° C.was favorable for TiO₂ coated hollow glass bead synthesis consideringthe energy cost and the diffusive solar light reflectance performance.

Table 1 also shows that TiO₂ coated hollow glass bead samples had higherdiffusive solar light reflectance than that of physical mixtures of TiO₂particles and hollow glass bead particles.

TABLE 1 Diffusive solar light reflectance of various samples DiffusiveSample solar reflectance (%) HGB@TiO₂-4.65/2:1/ethanol/room temperature89.61 HGB@TiO₂-4.65/2:1/ethanol/50 87.12 HGB@TiO₂-4.65/2:1/ethanol/8087.33 HGB@TiO₂-4.65/2:1/ethanol/110 90.08 HGB@TiO₂-4.65/2:1/ethanol/15089.10 HGB@TiO₂-4.65/2:1/water/room temperature 90.11HGB@TiO₂-4.65/2:1/water/50 90.64 HGB@TiO₂-4.65/2:1/water/80 92.09HGB@TiO₂-4.65/2:1/water/110 90.15 HGB@TiO₂-4.65/2:1/water/150 89.68HGB@TiO₂-3.1/2:1/ethanol/80 87.7 HGB@TiO₂-5.43/2:1/ethanol/80 90.5HGB@TiO₂-6.5/2:1/ethanol/80 90.8 HGB@TiO₂-4.65/8:1/water/80 99.96HGB@TiO₂-4.65/4:1/water/80 97.82 Physical mixture of HGB and TiO₂* 95.92Hollow glass bead 82.85 Amorphous TiO₂ 95.98 *TiO₂ concentration in thephysical mixture is the same as that of HGB@TiO₂-4.65/4:1/water/80. Thephysical mixture was made by mixing specific amounts of hollow glassbeads and amorphous TiO₂ according to the weight ratio of hollow glassbeads to TiO₂ in HGB@TiO₂-4.65/4:1/water/80.

In order to test the effect of TiO₂ on the density of TiO₂ coated hollowglass beads, the density of TiO₂ coated hollow glass beads was obtainedby separated mass and volume measurement. The results (Table 2) showthat TiO₂ coating can tune the density of TiO₂ coated hollow glassbeads.

TABLE 2 Density and TiO₂ content of TiO₂ coated hollow glass beadsamples Bulk TiO₂ density ρ concentration Sample (g/mL) (w %)ρ_(HGB@TiO2·)ρ_(HGB.) Hollow glass beads 0.1599 — — HGB@TiO₂-4.65/2:1/0.1955 20 1.223 ethanol/80 HGB@TiO₂-4.65/2:1/ 0.1954 20 1.222 water/80HGB@TiO₂-4.65/4:1/ 0.2658 50 1.662 water/80

Example 4—Characterization and Performance Test of Paint

Total solar light reflectance was tested using UV—VIS-NIRspectrophotometer UV-3600 (Shimadzu) with integrating sphere ISR 3100according to ASTM E903-96 and ASTM G159-98. Thermal conductivity wastested by using LFA 457 Microflash laser flash system (NETZSCH).Temperature difference test was conducted using the self-madetemperature difference test box (shown in FIG. 7). The temperature ofthe center of the test box was recorded during the test. The temperaturedifference (ΔT) between test boxes with reference test board(T_(reference)) and sample test board (T_(sample)) can be calculatedusing the following equation: ΔT=T_(reference)−T_(sample).

Table 3 shows the solar light reflectance of paint formulated with TiO₂coated hollow glass beads, according to the methods listed in Examples2a and 2b. The paint formulated with TiO₂ coated hollow glass beadsshows higher total solar light reflectance compared with that of paintformulated with hollow glass beads. When the concentration of HGB@TiO₂-4.65/2:1/water/80 was 7.4 w %, and the volume concentration of HGBwas the same as that of HGB @TiO₂-4.65/2:1/water/80 in paint, the totalsolar light reflectance increased from 75.08% to 79.93%. The paintformulated with TiO₂ coated hollow glass beads showed higher total solarlight reflectance compared with that of paint formulated with TiO₂ andhollow glass beads physical mixture. When the concentration ofHGB@TiO₂-4.65/2:1/water/80 was 17.4 w %, and TiO₂ concentration in thephysical mixture was the same as that of HGB@TiO₂-4.65/2:1/water/80, thetotal solar light reflectance increased from 83.64% to 85.74%.

TABLE 3 Total solar light reflectance of cool paints formulated withTiO₂ modified hollow glass beads Total solar light Film reflectanceConcentration of Coating thickness Sample (%) pigment (w %) times (mm)Original binder 48.92 — 4 0.916 Hollow glass beads 75.08 * 4 1.000HGB@TiO₂- 79.93 7.4^(a) 4 0.935 4.65/2:1/water/80 HGB@TiO₂- 81.55 11.8 30.800 4.65/2:1/water/80 HGB@TiO₂- 84.85 15.9 3 0.880 4.65/2:1/water/80HGB@TiO₂- 85.19 16.8 3 0.860 4.65/2:1/water/80 HGB@TiO₂- 85.74 17.4 30.860 4.65/2:1/water/80 HGB TiO₂ physical 83.64 17.4 3 0.850 mixture^(#)HGB@TiO₂- 86.27 18.9 3 0.860 4.65/4:1/water/80 HGB@TiO₂- 87.36 20 30.863 4.65/4:1/water/80 * the volume concentration of HGB in the paintis the same as that of the TiO₂ modified HGB in the paint^(a) ^(#)TiO₂concentration in the physical mixture is the same as that ofHGB@TiO₂-4.65/2:1/water/80

In order to test the effect of TiO₂ coated hollow glass beads on thethermal conductivity of paint, paint was prepared according to themethod of Example 2c. Table 4 shows that adding TiO₂ coated hollow glassbeads into the binder can decrease the thermal conductivity of thebinder by 76%. Paint formulated with TiO₂ coated hollow glass bead showslower thermal conductivity than that of paint formulated with physicalmixture of TiO₂ and hollow glass bead.

TABLE 4 Thermal conductivity of cool paint Concentration Sample (namepaint Thermal conductivity of with pigment utilized) (W/m K) pigment (w%) Binder 0.559 NA Hollow glass bead* 0.098 * Physical mixture of hollowglass 0.150 16.8 bead and TiO₂ ^(#) HGB@TiO₂-4.65/2:1/water/80 0.13316.8 *The volume concentration of hollow glass bead in the binder is thesame as that of HGB@TiO₂-4.65/2:1/water/80 in the binder ^(#)Theconcentration of TiO₂ in pigment composed of physical mixture of TiO₂and hollow glass bead is the same as that in TiO₂ coated hollow glassbead HGB@TiO₂-4.65/2:1/water/80

In order to test the cooling performance of paint formulated with TiO₂coated hollow glass beads, a temperature difference test was conductedunder sunlight irradiation. The results are shown in Table 5.

TABLE 5 Temperature difference test for various formulated paints withdifferent pigments^(#) Cooling Sample 1 Sample 2 performance (° C.)*Weather condition Cement board Cement board coated 8.7 Date: 2nd Augustwith paint which is 2015 formulated with Time: 12:30 PM- TiO₂ coatedhollow 15:10 PM glass bead^(a) Address: 3 Research Link, SingaporeTemperature: 31-33° C. Feels like: 37-38° C. Wind: 16 km/h- 23 km/hHumidity: 66-55% Cement board coated Cement board coated 1.2 Date: 8thOctober with paint which is with paint which is 2015 formulated withformulated with Time: 11:10 AM- TiO₂ and hollow TiO₂ coated hollow 1:00PM glass bead physical glass bead Address: 1 mixture^(b) FusionopolisPlace, Singapore Temperature: 32° C. Feels like: 36° C. Wind: 11 km/hHumidity: 55% Cement board coated Cement board coated 1.5 Date: 8thOctober with paint which is with paint which is 2015 formulated withformulated with Time: 1:30 PM-2:15 hollow glass bead^(c) TiO₂ coatedhollow PM glass bead Address: 1 Fusionopolis Place, SingaporeTemperature: 32° C. Feels like: 35° C. Wind: 13 km/h Humidity: 52%*Cooling performance = T_(sample 1)-T_(sample 2), T is the inner spacetemperature of test box with corresponding test board; ^(#)TiO₂ coatedhollow glass bead is HGB@TiO2-4.65/2:1/water/80, hollow glass bead isK25, TiO₂ in physical mixture of TiO₂ and hollow glass bead is amorphousTiO₂ ^(a)the concentration of TiO₂ coated hollow glass bead is 6 w %;^(b)The concentration of hollow glass bead and TiO₂ are the same as thatof TiO₂ coated hollow glass bead in ^(a); ^(c)The volume concentrationof hollow glass bead is the same as that of TiO₂ coated hollow glassbead in^(a)

The results show that paint formulated with cool pigment developed inthis application showed the highest cooling performance. The cool paintcan decrease the room temperature of test box roofed with cement boardby 8.7° C. The cooling performance of cool paint formulated withas-prepared cool pigment was also compared with that of the paintformulated with hollow glass bead only and physical mixture of hollowglass bead and TiO₂, respectively. The results show that the coolingperformance of cool paint formulated with as-prepared cool pigment inthis application was at least 1.2° C. higher than that of hollow glassbead and physical mixture of hollow glass bead and TiO₂. These resultssuggest the strong cooling performance of cool pigment developed in thisapplication.

Example 5—Coating on Hydrophobic Hollow Glass Beads

Here, TiO₂ is also coated onto the surface of hydrophobic hollow glassbead (NIPO PTE LIMITED of Singapore) without obvious freestanding TiO₂agglomerate formation.

FIG. 8(a) shows the SEM image of the original (uncoated) hydrophobichollow glass bead while FIG. 8(b) shows the SEM image of the sampleHGB(hydrophobic)@TiO₂-4.96/2:1/ethanol/80. This shows that the TiO₂ wasable to form a uniform coating on the surface of hydrophobic hollowglass beads without any free standing TiO₂ agglomerate formation. Thisis also confirmed by the elemental mapping image in FIG. 9(a) and FIG.9(b). FIG. 10 shows the dispersion of (a) original hydrophobic hollowglass beads in water and (b) TiO₂ coated hydrophobic hollow glass beadin water. The dispersion of hydrophobic hollow glass beads in water wasincreased due to the TiO₂ coating.

These results suggest the wide application scope of this application tocoat TiO₂ onto the surfaces of both hydrophilic and hydrophobic hollowglass beads.

FIG. 11 shows the XRD pattern of TiO₂ coated hollow glass beads (thatare hydrophobic). FIG. 11 shows that after subtracting the XRD patternof hollow glass bead and substrate, TiO₂ coated hollow glass beads doesnot show any peak, suggesting the amorphous structure of TiO₂ in theTiO₂ coated hollow glass beads.

Comparative Example

The titania-coated hollow glass beads were compared against a number ofmarket products. In market, most cool paints focus on utilization ofonly solar light reflectance property and some paint products have bothsolar light reflectance and low thermal conductivity properties. Someproducts from companies in the market which show the certifiedproperties are shown in Table 6.

TABLE 6 Comparison with market products Total solar light Thermalconductivity TiO₂ content Product reflectance (%) (W/mK) (w %) CompanyFECOAT 1000 85* No thermal  7-10^(#) BASF UF 1001 WHITE insulationproperty mentioned Thermoshield 84^($) 0.142^($) 10-30^($) ThermoshieldWhite Australia Pty Ltd HGB @ TiO₂- 85.19 0.133 3.4 — 4.65/2:1/water/80*The data is obtained from Energy Star(https://www.energystar.gov/productfinder/product/certified-roofproducts/?scrollTo=103&search_text=&energy_star_partner_isopen=1&brand_name_isopen=&zip_code_filter=&product_types=Select+a+Product+Category&energy_star_partner_filter=BASF+Corporation)^(#)FECOAT 1000 UF 1001 WHITE Safety Datasheet 2015, version 3.1 ^($)Thedata is obtained from company website:http://www.thermoshield.com.au/technical-data.html

Comparing market products with the TiO₂ coated hollow glass beads of theapplication, when the total solar light reflectance is almost the same,the content of TiO₂ used in this application is much lower. Comparingthe disclosed cool paint with Thermoshield White, the thermalconductivity of the disclosed cool paint is lower. It is commonknowledge in industry that using high content of TiO₂ will induce hightotal solar light reflectance, however, TiO₂ is expensive and possessesa large carbon footprint. In this application, TiO₂ is coated uniformlyonto the surface of hollow glass bead without any obvious free standingagglomerates, which will make full use of the effective interfacialsurface area. This therefore results in achieving high total solarreflectance with lower content of TiO₂.

INDUSTRIAL APPLICABILITY

The disclosed method may be able to form titania-coated inorganicparticles that are used in a formulation. The titania-coated inorganicparticles may be used in a paint formulation to impart desiredproperties such as high solar light reflectance, low thermalconductivity and/or high emissivity.

The disclosed method may avoid the problem of freestanding agglomeratesof titanium dioxide particles. The disclosed method may not requirecontrolling the crystal phase of titanium dioxide. The disclosed methodmay control the rate of TiO₂ coating formation by controlling the ratiobetween the titania precursor, inorganic particles and water. Thedisclosed method may not require the use of pH control or temperaturecontrol. The disclosed method may not require the use of pre-heating orcomplicated post-treatment steps.

It will be apparent that various other modifications and adaptations ofthe invention will be apparent to the person skilled in the art afterreading the foregoing disclosure without departing from the spirit andscope of the invention and it is intended that all such modificationsand adaptations come within the scope of the appended claims.

What is claimed is:
 1. A method for forming a titania-coated inorganicparticle comprising: a. stirring a mixture of a titania precursor and aninorganic particle in an organic solvent for a time period of more than1 hour to cause adsorption of said titania precursor on the surface ofsaid inorganic particle, and b. adding water to the mixture understirring to convert said titania precursor to titania which then forms acoating on said inorganic particle, wherein said organic solvent ismiscible with an aqueous solution and said organic solvent is analcohol; and wherein in operation (b), said water and said titaniaprecursor are present at a molar ratio of water to titania precursor inthe range of 2:1 to 8:1.
 2. The method according to claim 1, wherein inoperation (a), said titania precursor and said inorganic particle arepresent at a ratio of titania precursor (molar) to inorganic particle(g) in the range from 8:1 to 18:1 (mmol/g).
 3. The method according toclaim 1, wherein said titania precursor is a titanium alkoxide.
 4. Themethod according to claim 3, wherein said titanium alkoxide is atitanium C₁₋₁₀-alkoxide.
 5. The method according to claim 4, whereinsaid titanium alkoxide is a C₁₋₄-alkoxide selected from the groupconsisting of titanium methoxide, titanium ethoxide, titaniumisopropoxide and titanium butoxide.
 6. The method according to claim 1,wherein said inorganic particle is a glass particle selected from thegroup consisting of a silica glass particle, a soda-lime glass particle,a borosilicate glass particle, an aluminosilicate glass particle andmixtures thereof or wherein said glass particle is a hollow glassparticle comprising a glass shell encapsulating an inner void.
 7. Themethod according to claim 1, wherein said inorganic particle is amicrosphere.
 8. The method according to claim 1, wherein said water isadded to said stirred mixture in the form of water droplets.
 9. Themethod according to claim 1, wherein in operation (b), said stirring isundertaken for more than an hour.
 10. The method according to claim 1,wherein said method is undertaken at neutral pH.
 11. The methodaccording to claim 1, wherein the method further comprises: c.separating the titania-coated inorganic particle from the mixture andoptionally the operation of: d. drying the titania-coated inorganicparticle at a temperature in the range of 25° C. to 100° C.